In David Eller's excellent book, Atheism Advanced, Eller basically explains morality as those moral rules made up by people in order to define what it means to be part of any culture. They are usually based upon the religious myths each culture accepts. There is no morality then, only "moralities."

He finds that there are moralities among animals like Chimps, so it shouldn't surprise us when language bearing humans came up with more elaborate moral rules. And since we're talking about human beings, it's no surprise that our moralities have some major similarities since we are social animals who need to get along, to be loved and to love, to help and to be helped. Anyone who doesn't accept the moral rules of a culture are not allowed in the group, or we banish them, ostracize them, imprison them, and kill them. Do you want the benefits of being in the group? Then obey the moral rules, or at least don't get caught. Otherwise, you’re on your own. As such, there is nothing prohibiting someone from not accepting the moral rules of a culture if s/he doesn't want the benefits of the group (which would be a Freudian "death wish"). Are acts like murder, rape, and theft objectively and universally "wrong" then? That's probably a nonsensical question.

Therefore, there can be no argument for the existence of God based on morality. Human beings make up their own moralities because we're social beings who need to belong and get along. Morality is part of our survival instinct. We need other people to survive!

----------For a Christian who might be stunned by the conclusion that it's probably a nonsensical question whether or not murder, rape, and theft are objectively and universally "wrong," then think again. Look at your own Bible. There is plenty of that to be found in it, all sanctioned by your barbaric God. Elsewhere I've argued that rational self-interest can account for our morality.